1. Field of the Invention
The invention relates generally to reverse osmosis and ultrafiltration fluid separation processes, and is applicable particularly to water desalination and purification by reverse osmosis.
2. Prior Art
Desalination by reverse osmosis is achieved by pumping a feed stream of saline water at an elevated working pressure into a pressure resistant vessel containing an array of semipermeable membranes. Purified product water of greatly reduced salinity permeates across the membranes into low pressure collection channels if the working pressure exceeds feed stream osmotic pressure. Considerable excess working pressure above the feed stream osmotic pressure is required to produce sufficient product water flux across membranes of reasonable surface area, and also to ensure sufficient dilution of the small but finite salt diffusion through the membrane which always exists when there is a concentration gradient across such membranes. For sea water whose osmotic pressure is about 25 Kg/sq. cm, typical working pressure for single stage reverse osmosis is in the order of 70 Kg/sq. cm.
While some of the feed stream permeates through the membranes, the balance becomes increasingly concentrated with salt rejected by the membranes. In a continuous reverse osmosis process, a concentrate stream must be exhausted from the vessel to prevent excessive salt accumulation. In sea water desalination, this concentrate stream may be typically 70% and sometimes as much as 90% of the feed stream. The concentrate stream leaves the vessel at almost full working pressure, but before the concentrate stream is exhausted from the apparatus, it must be depressurized. In common reverse osmosis apparatus the concentrate stream is depressurized by throttling over a suitable back pressure valve, for example a restrictor valve, which regulates the working pressure while dissipating all the pressure energy of the concentrate stream. It is known to recover some of the concentrate stream pressure energy using recovery turbine devices, however such energy recovery devices have mostly seemed practicable only for large stationary plants where efficiency and economy advantages of scale would apply.
Without energy recovery devices, small scale manually operated reverse osmosis desalinators for use in households, life-boats, etc. would be almost unpracticable. Similarly, using wind power for desalination is discouraged by high energy consumption.
Furthermore, for high recovery concentration polarization must be controlled. Concentration polarization in the feed stream is the tendency for a concentration gradient to develop in the feed stream with high salt concentration on the membrane face during reverse osmosis. This tendency results from the bulk transport of saline feed water toward the membrane face and the accumulation of salt in the boundary layer as less saline water permeates through the membrane, balanced by diffusion of salt back out of the boundary layer. Concentration polarization is detrimental especially with feed solutions of high osmotic pressure such as sea water, because the membrane sees a higher concentration which raises the effective osmotic pressure. When concentration polarization occurs, working pressure for given product flux must be increased, product salinity will be increased, and membrane life may be impaired.
Reverse osmosis systems are typically designed to reduce concentration polarization effects by forced convection through the membrane array. Forced convection may be provided by circulating a low ratio of product flow to concentrate flow through suitably configured feed channels between the membrane faces, or by auxiliary recirculation or mechanical stirring devices. It is essential that continuous feed circulation be maintained through the membrane array, because even momentary stagnation of flow may cause severse concentration polarization.
Operation at low ratios of product flow to concentrate flow is also generally favourable to the reduction of concentration polarization effects, but of course increases the feed pumping energy expenditure for given product flow delivery.